[racket] Whalesong 1.7 released

Here's a list of changes in Whalesong 1.7:
* Added primitives: hash-keys, hash-values, hash-for-each, hash-map, write.
* Merged some changes to support sk's cs019 language level.
I didn't make a separate release note for 1.6. In 1.6, Whalesong
constraints its use of the stack to support iOS 5 platforms better.
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I'll probably be making some point updates as this week progresses in
response to feedback from Shriram's class. Also, I'll need to fix
Whalesong's bytecode parser to work with Racket 5.2's, which I
understand is coming out shortly.